The isolator functioning to permit signals to pass in a forward direction while preventing signals from passing in a reverse direction is used to remove reverse-direction signals from a communications apparatus. In a cell phone, for example, radiation efficiency is improved by utilizing radiation from a metal portion of a casing to increase an apparent antenna volume, but impedance varies under strong influence of adjacent human bodies, resulting in the likelihood that part of output signals are reflected by an antenna to generate reverse-direction signals. When such reverse-direction signals are input to a power amplifier directly, power efficiency is lowered, and noise is generated. Accordingly, an isolator is disposed between the antenna and the power amplifier. Such isolator comprises a magnetic body (microwave ferrite such as garnet), pluralities of central conductors crossing the magnetic body, and a permanent magnet applying a DC magnetic field to the magnetic body to generate a rotating resonance magnetic field therein.
FIG. 2 shows the equivalent circuit of a non-reciprocal circuit device called two-port isolator, which is disclosed in JP 2004-15430 A, and FIG. 28 shows the structure of the non-reciprocal circuit device. This two-port isolator comprises a first input/output port P1, a second input/output port P2, a first inductance element Lin and a first capacitance element Ci connected between both input/output ports P1, P2 for constituting a first parallel resonance circuit, a resistance element R connected in parallel to the first parallel resonance circuit, and a second inductance element Lout and a second capacitance element Cf connected between the second input/output port P2 and a ground for constituting a second parallel resonance circuit. In the two-port isolator, the first parallel resonance circuit sets a frequency for providing the maximum isolation (attenuation in a reverse direction), and the second parallel resonance circuit sets a frequency providing the minimum insertion loss.
The first inductance element Lin and the second inductance element Lout are constituted by a first central conductor Lin and a second central conductor Lout formed by strip-shaped conductors and crossing on a main surface side of a ferrite plate to which a DC magnetic field is applied from the permanent magnet 30. A part comprising the magnetic body and the first and second central conductors is called a central conductor assembly 4.
In this example, the first capacitance element Ci and the second capacitance element Cf are constituted by electrode patterns in a multi-layer ceramic substrate 10. The multi-layer ceramic substrate 10 is provided on a main surface with an electrode pad 15 and connecting pads 17, 18. The electrode pad 15 is connected to a terminal electrode P2 of the second central conductor Lout formed on a side surface of the multi-layer ceramic substrate 10 through a via-hole electrode and a side surface electrode. The connecting pad 17 is connected to a terminal electrode P1 of the first central conductor Lin formed on a side surface of the multi-layer ceramic substrate 10 through a via-hole electrode and a side surface electrode. The connecting pad 18 is connected to ground electrodes GND through via-hole electrodes and side surface electrodes. As the first capacitance element Ci and the second capacitance element Cf, multi-layer chip capacitors, and single-layer capacitors formed on upper and lower surfaces of a dielectric substrate may be used. The permanent magnet 30, the central conductor assembly 4 and the multi-layer ceramic substrate 10 are received in upper and lower cases 22, 25 made of a magnetic metal.
According to the miniaturization of cell phones and increase in the number of their parts due to multi-functionalization, there is strong demand to make isolators smaller. Although isolators having external sizes of 3.2 mm×3.2 mm×1.2 mm or 3.2 mm×2.5 mm×1.2 mm are widely used at present, there is further demand for smaller isolators of 2.0 mm×2.0 mm×1.1 mm, for example. According to such miniaturization, central conductor assemblies constituting two-port isolators should be miniaturized.
Conventionally proposed are central conductor assemblies having various structures, for example, a central conductor assembly comprising a copper foil wound around a ferrite plate; a laminate-type central conductor assembly (disclosed in JP 9-232818 A) obtained by laminating pluralities of dielectric sheets printed with electrode patterns for the central conductors and integrally sintering them as shown in FIG. 29, etc.
To obtain a small isolator of 2.0 mm×2.0 mm, a central conductor assembly should have an external size reduced to about 1.5 mm×1.2 mm. The miniaturization of the central conductor assembly results in the decreased volume of a magnetic body and shorter central conductors, providing the central conductors with smaller inductance. Accordingly, capacitance elements should have large capacitance to achieve resonance at a desired frequency, which is difficult because of the miniaturization of the non-reciprocal circuit device. As a result, the input/output impedance deviates from the impedance of an external circuit to cause mismatching, likely resulting in deteriorated insertion loss, a shrunken passband width, etc.
To cope with impedance deviation, an impedance-matching circuit is connected to an input/output port of the non-reciprocal circuit. FIG. 27 shows an example in which a matching circuit 90 is arranged on the side of a first input/output port P1. Capacitance element Cz is connected when the input impedance is inductive, and an inductance element is connected when the impedance is capacitive. However, the addition of a matching circuit increases the number of parts, hindering the miniaturization of the non-reciprocal circuit device.